The present invention relates to an image forming apparatus such as a printer, a facsimile or a copying machine which forms an image by using an electrophotographic method. More particularly, the invention relates to a technique for taking a countermeasure against ozone generated from a corona charger.
In general, an image forming apparatus using an electrophotographic technique comprises: a photosensitive body having a photosensitive layer on an outer peripheral surface; a charger for uniformly charging the outer peripheral surface of the photosensitive body; an exposer for selectively exposing the uniformly charged outer peripheral surface, thereby forming an electrostatic latent image; a developer for giving a toner serving as a developing agent to the electrostatic latent image, thereby forming a visible image (a toner image); a transferer for transferring the toner image developed by the developer onto a recording medium such as paper; and a fuser for fixing the toner image formed on the recording medium with heat.
For the charger, a corona charger referred to as a scorotron charger is utilized generally.
The scorotron charger is constituted by a discharge electrode, a support member for supporting the discharge electrode, a back plate for carrying out a stable discharge, and a grid for controlling a charging potential on a photosensitive body. When actually carrying out the charging, for example, a voltage of −4 kV to −6 kV is applied to the discharge electrode, a voltage of −600 V (depending on a desired potential to be actually charged) is applied to the grid, and the back plate is set to have the same potential as that of an earth or the grid so that a corona discharge can be generated by the discharge electrode and the surface of the photosensitive body can be charged to approximately −600 V. Since such a charger can uniformly carry out the charging over the photosensitive body with a simple structure, it is widely used.
However, it has been known that such a corona charger utilizes the discharge, and therefore, a discharge product such as ozone or NOx is generated and the discharge product causes a deterioration in an image when the discharge product sticks to the photosensitive body or the charger.
Therefore, a conventional image forming apparatus is provided with an opening for sending air in the axial direction of the back face of the charger and a duct for sending the air to the back face of the charger, for example, and sends the air from one of the ends of the duct, thereby discharging ozone in the charger. Such a structure is disclosed in Japanese Utility Model Publication No. 6-43815U, for example.
Moreover, there has also been known an image forming apparatus in which a fan is provided for efficiently collecting the ozone generated by the charger from a portion between a cleaning blade and a charge removing lamp, and the fan is rotated to cause the ozone generated from the charger to flow and to deliver the same ozone to a waste toner bottle disposed in the rear portion of the apparatus through a pipe connected to the fan. The ozone is thus caused to come in contact with a waste toner stored in the waste toner bottle and is decomposed. Such a configuration is disclosed in Japanese Patent Publication No. 5-216321A.
In the case in which continuous image formation is carried out by the image forming apparatus using the corona charger, ozone is apt to remain in the vicinity of the corona charger also after the end of the image formation.
For this reason, in the conventional image forming apparatus described above, it is necessary to continuously send the air in order to discharge the ozone also after the end of the image formation.
Accordingly, there has been a problem in that a noise made by the fan or the like operated irrespective of the end of the image formation gives an unpleasant feeling to a user. Moreover, there has also been a problem in that the standby power of the apparatus is also increased, which is not preferable in respect of energy saving.
Furthermore, the ozone is heavier than the air. In an arrangement in which a charger is provided in the upper part of a photosensitive body, therefore, there has also been a problem in that an opportunity for the ozone going downward by a self-weight to come in contact with the photosensitive body is increased, resulting in the promotion of a deterioration in the photosensitive body.
On the other hand, there has been know a fuser using a pair of rollers. In such a case, when a toner image is to be fixed by the fuser, a volatile component contained in a toner is volatilized and the volatile substance is diffused to contaminate the discharge electrode of the corona charger, so that uniform discharge is hindered.
There has been known a conventional image forming apparatus comprising: a duct for sucking air containing ozone generated from a charger and feeding back the air having the ozone removed therefrom to the charger again in order to prevent the ozone generated in the charger from moving out of the image forming apparatus to attain a harmlessness; at least one fan for generating air flow in the duct; and an ozone removing member for removing the ozone in the duct. Such a configuration is disclosed in Japanese Patent Publication No. 2000-149018A. In such an image forming apparatus, the duct including the charger and the air flow generator are constituted as a closed vent path. Therefore, it is possible to prevent the volatile substance from entering the duct, therefore, the charger.
In both of the image forming apparatuses disclosed in Japanese Utility Model Publication No. 6-43815U and Japanese Patent Publication No. 5-216321A, a duct including a charger and an air flow generator are not constituted as a closed vent path. Therefore, there is an anxiety that the charger might be contaminated by a volatile substance generated in a fuser, resulting in non-uniform charging.
In both of the image forming apparatuses disclosed in Japanese Utility Model Publication No. 6-43815U and Japanese Patent Publication No. 2000-149018A, there is a possibility that the ozone might stay at the downstream side of an air flow in the corona charger and an ozone product might severely stick locally. In particular, the amount of H2O to be a carrier for a corona discharge is small after the execution of the continuous printing at a low temperature and a low humidity in a state in which the discharge electrode of the corona charger is old. Consequently, the corona discharge is apt to be unstable and the influence of the ozone product sticking is remarkable so that an image defect or a change in a density which cannot be disregarded is generated in some cases.